Method of manufacturing an imager and imager device

ABSTRACT

Techniques are discloses regarding methods of manufacturing an imager as well as an imager device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application a divisional application of U.S. patentapplication Ser. No. 16/566,171, filed on Sep. 10, 2019, issued as U.S.Pat. No. 10,692,921, which is a divisional application of U.S. patentapplication Ser. No. 15/586,498, filed on May 4, 2017, issued as U.S.Pat. No. 10,411,060, which is a continuation application of U.S. patentapplication Ser. No. 14/186,390, filed on Feb. 21, 2014, issued as U.S.Pat. No. 9,659,992, which claims the benefit of the filing date ofprovisional U.S. Patent Application No. 61/803,998, filed on Mar. 21,2013, the contents of each of which are incorporated herein by referencein their entireties.

BACKGROUND

Imagers are used in many applications nowadays. This includes 2D CMOSimagers as well as 3D imagers such as 3D depth imagers (3D cameras)which may be based for example on the ToF principle (time-of-flightprinciple) or other principles. 3D cameras may provide human gesturerecognition in natural user interfaces or passenger recognition forautomotive safety functions. Distinguished from 2D cameras, 3D camerasfor example provide an array of pixel in which each pixel is capable toprovide information related to a distance of the object captured by thepixel. Such information may for example be based on a time of flight oflight reflected from an object captured by the pixels.

With the implementation of increasing number of pixels on a depth imagerchip and the shrinking of pixel sizes going along therewith, the needexist for a concept which allows efficient conversion of light intocharge carriers and efficient controlling of the charge carriers in eachpixel.

In view of the above it would be beneficial to have a concept which iscapable of providing a high degree of efficiency for imagers. Inaddition, it would be beneficial to have a concept which allows theparallel processing of control electrodes in the optical sensitive areasas well as transistors for an integrated circuit provided for furthersignal processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a depth imager system according to anembodiment;

FIG. 2A shows a schematic cross-sectional view according to anembodiment;

FIG. 2B shows a diagram of an operation according to an embodiment;

FIGS. 3A to 3G show schematic cross-sectional views according to anembodiment;

FIGS. 4A to 4I show schematic cross-sectional views according to anembodiment;

FIGS. 5A to 5C show schematic cross-sectional views according to anembodiment;

FIGS. 6A and 6B show schematic cross-sectional views according to anembodiment;

FIGS. 7A and 7B show schematic cross-sectional views according to anembodiment;

FIG. 8 shows doping profiles according to embodiments; and

FIG. 9 shows a diagram of Bor concentrations in silicon versus Borconcentrations in a Bor-doped glass layer.

DETAILED DESCRIPTION

The following detailed description explains exemplary embodiments. Thedescription is not to be taken in a limiting sense, but is made only forthe purpose of illustrating the general principles of embodiments whilethe scope of protection is only determined by the appended claims.

In the exemplary embodiments shown in the drawings and described below,any direct connection or coupling between functional blocks, devices,components or other physical or functional units shown in the drawingsor described herein can also be implemented by an indirect connection orcoupling unless otherwise noted. Functional blocks may be implemented inhardware, firmware, software, or a combination thereof.

Further, it is to be understood that the features of the variousexemplary embodiments described herein may be combined with each other,unless specifically noted otherwise.

In the various figures, identical or similar entities, modules, devicesetc. may have assigned the same reference number. Example embodimentswill now be described more fully with reference to the accompanyingdrawings. Embodiments, however, may be embodied in many different formsand should not be construed as being limited to the embodiments setforth herein. Rather, these example embodiments are provided so thatthis disclosure will be thorough and complete, and will fully convey thescope to those skilled in the art. In the drawings, the thicknesses oflayers and regions are exaggerated for clarity.

In the described embodiments, various specific views or schematic viewsof elements, devices, features, etc. are shown and described for abetter understanding of embodiments. It is to be understood that suchviews may not be drawn to scale. Furthermore, such embodiments may notshow all features, elements etc. contained in one or more figures with asame scale, i.e. some features, elements etc. may be shown oversizedsuch that in a same figure some features, elements, etc. are shown withan increased or decreased scale compared to other features, elementsetc.

It will be understood that when an element is referred to as being “on,”“connected to,” “electrically connected to,” or “coupled to” to anothercomponent, it may be directly on, connected to, electrically connectedto, or coupled to the other component or intervening components may bepresent. In contrast, when a component is referred to as being “directlyon,” “directly connected to,” “directly electrically connected to,” or“directly coupled to” another component, there are no interveningcomponents present. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

It will be understood that although the terms first, second, third,etc., may be used herein to describe various elements, components,regions, layers, and/or sections, these elements, components, regions,layers, and/or sections should not be limited by these terms. Theseterms are only used to distinguish one element, component, region,layer, and/or section from another element, component, region, layer,and/or section. For example, a first element, component, region, layer,and/or section could be termed a second element, component, region,layer, and/or section without departing from the teachings of exampleembodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like may be used herein for ease of description todescribe the relationship of one component and/or feature to anothercomponent and/or feature, or other component(s) and/or feature(s), asillustrated in the drawings. It will be understood that the spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures.

The term vertical is used in a non-limiting way to describe inembodiments directions vertical or substantially vertical to a surfaceof a substrate such as a main surface of the substrate. The term lateralis used in a non-limiting way to describe in embodiments directionsparallel or substantially parallel to a surface of a substrate such as amain surface of the substrate.

The term substrate used in embodiments may include but is not limited tosemiconductor substrates such as a semiconductor die, a stacked die, asemiconductor die with one or more additional semiconductor layers suchas epi-layers, polysilicon layers etc.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, and/or components.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andshould not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The embodiments described below are directed to a new manufacturingconcept for a depth imager based on the time of flight (TOF) principlesuch as for example an imager based on the photon mixing principle.Depth imagers may include only one pixel or an array of pixel fordetermining distances.

In depth imagers based on the photonic mixing principle a phase ofreflected light is determined by transmitting modulated light to anobject and capturing in each pixel of a pixel array the phaseinformation by mixing the reflected light with a demodulation signal ofhaving the same frequency as the modulated light.

FIG. 1 shows an embodiment of a TOF camera 100 based on the photonicmixing principle. Light generated by a light source 102 is inpredetermined time intervals continuously amplitude modulated based on amodulation signal 104 generated by a signal source 106. The modulationsignal may include a rectangular waveform, a sine waveform of othersignal waveforms. The modulated light signal is provided to determinethe distance to an object 108. The modulated light reflected by theobject 108 is directed to an imager device 110 which includes pixels asshown for example in the embodiments described with respect to FIGS. 1A,2A, 3, 4 and 5. In the imager device 110, a signal 104A whichcorresponds to the modulation signal 104 phase shifted by apredetermined phase, e.g. 0°, 90°, 180° and 270°, is provided to thecontrol electrodes for mixing and demodulation of the reflected lightwithin each pixel. Certain time intervals are assigned for each of thepredetermined phases. After integrating the signals in the respectivetime intervals for each phase 0°, 90°, 180° and 270°, output signals I0,I1, I2, and I3 are obtained corresponding to each phase. Based on theoutput signals I0, I1, I2, I3, the phase information corresponding tothe time to travel can be computed as is known to a person skilled inthe art. It is to be noted that the structure of FIG. 2A described belowhaving two read-out nodes at both sides allows to simultaneously obtainthe phases I0 and I2 and the phases i1 and I3, respectively.

In the embodiment shown in FIG. 1, the signal 104A is provided phaseshifted with respect to the modulation signal 104. It is to beunderstood that only the relative phase shift of the modulation signaland the demodulation signal is used for measuring the object distance.Therefore, in other embodiments a system with interchanged signals 104and 104A may be provided in which the modulation signal 104 for thelight modulation is phase shifted in different time intervals withrespect to the signal 104A which is provided with no phase change.

FIG. 2A shows an embodiment of a depth imager device 200. FIG. 2A showsa semiconductor substrate 202 having a photo-conversion region 212. Thedevice is configured such that light penetrates into thephoto-conversion region 212 and at least a part of the incoming light isconverted into charge carriers of both types, i.e. electrons and holes.In some embodiments, the light might be introduced to thephoto-conversion region from a front side 200A of the device 200. Inother embodiments, the light might be introduced to the photo-conversionregion from the back side 200B of device 200. The photo-conversionregion may in embodiments extend in vertical direction between 15 μm and60 μm into the substrate with each value in this range and eachsub-range constituting a specific embodiment.

The device 200 further includes an electric insulating layer 204 ofinsulating material above which a plurality of control electrodes, forexample a first control electrode 206 a, a second control electrode 206b and a third control electrode 206 c are provided. The electricinsulating layer 204 is typically provided as thin layer, e.g. a thingate layer, and may for example include silicon oxide. The elementsshown in FIG. 2A show a simplified view of a single pixel of theplurality of pixels typically provided in a depth imager. The controlelectrodes may form in embodiments the modulation gates of ademodulation structure within a pixel of a continuous wavetime-of-flight imager. It is therefore to be noted that in someembodiments multiple pixels are formed within the device resulting in arepetition of the structures and elements shown in FIG. 2A for a singlepixel.

Spacers 208 are formed at the side walls of the control electrodes 206a, 206 b, 206 c. The device 200 may include additional structures whichare not shown in FIG. 2A including for example an additional layer forprotection of the semiconductor substrate 202 during a spacer etchingand further layer deposition, lens structures, passivation structures onthe metal stack and reflection structures which will be describedfurther below in more detail.

As can be observed in FIG. 2A, a cover layer 210 extending above thesemiconductor substrate 102 is provided to cover the control electrodes206 a, 206 b, 206 c and spacers 208 as well as other structures. FIG. 2Afurther shows read out nodes 214 at the lateral side of the pixel inorder to collect and read out charges transferred to the respective readout nodes by the control electrodes 206 a, 206 b, 206 c. The read outnodes 214 may be formed in embodiments by a p-n junction regionestablished by providing for example a n-type doped well in a p-typedoped substrate 202. The read out nodes 214 are electrically coupled tofurther circuit elements by a conductive structure 218 in order to allowfurther processing of the pixel signals.

In the embodiment of FIG. 2A, the second control electrode 206 b isarranged between the first control electrode 206 a and the third controlelectrode 206 c with respect to a lateral direction (such as thex-direction shown in FIG. 2A). In some embodiments, the second controlelectrode has a lateral distance to at least one of the first or secondcontrol electrodes between 50 nm and 1 μm, where each value within thisrange and each sub-range forms an embodiment. In some embodiments, thethird control electrode has a lateral distance to at least one of thefirst or second control electrode between 0.1 μm and 0.5 μm. In someembodiments, the second control electrode has a lateral distance to bothof the first or third control electrode between 50 nm and 1 μm, whereeach value within this range and each sub-range constitutes oneembodiment. In some embodiments, the second control electrode has alateral distance to both of the first or third control electrode between0.1 μm and 0.5 μm.

In embodiments, the control electrodes 206 a, 206 b and 206 c aremanufactured such that the control electrodes are transparent or atleast semi-transparent to the incoming light generating the chargecarriers in the photo-conversion region 212. This may be provided byhaving a respective thin layer for the control electrodes and/or usingmaterial which is transparent or semi-transparent for the incominglight. In some embodiments, the light of operation may be infrared lightor near-infrared light. In some embodiments, the light of operation maybe visible light.

In embodiments, the control electrodes 206 a, 206 b, 206 c areelectrodes to direct the photo-generated charge carriers in a lateraldirection towards the read out nodes 214 based on the electricpotentials present at the respective control electrodes 206 a, 206 b,206 c. In embodiments, the electric potentials at the control electrodes206 a, 206 b, 206 c causes the generating of time-varying space chargeregions in semiconductor regions below the respective control electrode.As will be described further below, the device 200 is capable togenerate space charge regions of different extensions below therespective control electrodes 206 a, 206 b, 206 c based on therespective electric potential present at the control electrode. Below arespective control electrode, the extension of the space charge regionand therefore the electric potential is approximately constant while inthe region between two adjacent control electrodes with different spacecharge extensions electric drift fields are generated. As a result apotential distribution for photo-generated charge carriers is generatedin the semiconductor region near the substrate surface causing driftfields in a lateral direction depending on the electric potentialspresent at the control electrodes 206 a, 206 b, 206 c.

FIG. 2B shows a diagram resulting from an example operation of thecontrol electrodes 206 a, 206 b and 206 c of FIG. 2A according to anembodiment. FIG. 2B shows as abscissa the electric potentialdistribution as negative electric potential caused by the controlelectrodes 206 a, 206 b, and 206 c in the semiconductor regions belowthe control gates. The negative electric potential corresponds to thepotential energy seen by photo-generated electrons (negative charges)which are in the described embodiments the charge carriers to betransferred by the control electrodes 206 a, 206 b, 206 c.

As can be seen, the potential applied to the first control electrode 206a (indicated in FIG. 1B by “a”) varies such that a maximum of thepotential energy is obtained at time t=0 and a minimum is obtained attime t=T/2. The second control electrode 206 b (indicated in FIG. 1B by“b”) is kept constant. It can be seen at both times t=0 and t=T/2, thepotential distribution is a step-wise such that for the controlelectrode 206 b arranged between the control electrodes 206 a and 206 cthe potential of the control electrode 206 b is also between thepotential of the control electrodes 206 a, 206 c. The use of two controlelectrodes driven by time-varying signals while one of the controlelectrodes is maintained at a constant electric potential provides therequired step-wise potential distribution in a very power-efficientmanner since no time-varying driving signal has to be applied to thecontrol electrode 206 b.

It is to be noted that the number of steps in the potential distributiongenerated by the control electrode configuration can be increased byadding additional control electrodes. If the number of controlelectrodes is K, the number of steps in the potential distribution addsup to K+1.

In the operation described above, the generated charge carriers may beguided by the electric field generated by the control electrodes to readout nodes which are arranged lateral to the control electrode structure.In other words, the control electrode arrangement is capable to providea bidirectional transfer to each lateral side once the charge carriersreach the influence zone of the control electrodes. In opticaltime-of-flight sensors the controlling of the charge carriers is timecritical since the operation depends on the timely transportation of thecharge carriers to the laterally arranged read-out nodes.

In conventional systems, charge carriers generated inside thephoto-conversion region move by diffusion from the origin of the chargegeneration to the influence zone of the control electrodes 206 a, 206 b,206 c. In some embodiments, doping profiles may be provided in thesubstrate 202 to generate build-in fields for providing acceleratedmovement of the charge carriers. In such embodiments, drift movement isprovided for the charge carriers. Only at the influence zone of thecontrol electrodes 206 a, 206 b, 206 c, the charge carriers are thensubjected to electric fields which cause the lateral transfer to theread out nodes 214 based on the corresponding potential distributionapplied to the control electrodes 206 a, 206 b, 206 c.

Referring now to FIGS. 3A-3G, an embodiment of manufacturing an imagersuch as the depth imager shown in FIG. 2A is described. It is to benoted that FIGS. 3A-3G show on the left hand a pixel region of thesubstrate which later forms a pixel of the imager such as the regionincluding control electrodes 206 a and 206 b and on the right hand sidea transistor region of the substrate in which one or more transistorsare formed in parallel to the pixel. The transistor region may includetransistor MOS-transistors, CMOS transistors or other types of gatecontrolled transistors as well as other transistors. It is also to beunderstood that the FIGS. 3A-3G are not drawn to scale but rather showschematically some elements and the way they are processed in acomprehensive manner. It is further to be understood that betweenconsecutive figures one processing step or multiple processing stepswhich are not shown may be applied, such as for example the depositionand structuring of a masks, an etching through a masks and a removal ofa masks.

FIG. 3A shows the substrate 202 with the electric insulating layer 204formed at a main surface 202A of the substrate 202. The electricinsulating layer 204 may include a single layer or a layer stack ofdifferent materials. In some embodiments, the electric insulating layer204 may comprise an oxide material such as a thin layer of siliconoxide. The electric insulating layer 204 may for example be formed by athermal oxide process applied to substrate 202 or other oxide processes.The electric insulating layer 204 provides in the finished imager anelectric isolation of the control electrodes in the pixel region toallow manipulation of photo-generated charge carriers at the substrateinterface and a gate isolator in the transistors region for transistoroperation.

It is to be noted here that the substrate 202 may be pre-processed toinclude additional structures, layers, etc. In other words, thesubstrate 202 may include not only a semiconductor substrate ofcrystalline material but for example also composite substrates,substrates with wells of different doping, additional layers, structuresetc. Typically, the substrate 202 may comprise a p-doped silicon butother doping types and substrate materials may be used in otherembodiments. An embodiment of providing a p-doping substrate isdescribed further below with reference to FIG. 9.

FIG. 3B shows the structure of FIG. 3A after a conductive layer 304 hasbeen deposited on the electric insulating material 204 which later formsthe control electrodes in each pixel of the imager device and the gateelectrodes in the transistor region.

FIG. 3C shows the conductive layer 304 of FIG. 3B after structuring intocontrol electrode structures 306A and 306B and gate electrode structures307A, 307B. The control electrode structures 306A and 306B in the pixelregion may for example correspond to the control electrodes 206A and206B shown in the depth imager device of FIG. 2A. While only two controlelectrode structures are shown, it is to be noted here that in additionto the control electrode structures 306A and 306B further controlelectrode structures may be formed in each pixel of the depth imagerdevice such as for example a control electrode structure to form acontrol electrodes 296A, 206B and 206C as shown in FIG. 2A.

Structuring the conductive layer 304 may include the generation of ahard mask, the structuring of the hard mask an etching through the hardmask to selectively remove the conductive layer 304 and a removing ofthe hard mask. Furthermore, as shown in FIG. 3C, the insulating layer204 may be removed in regions outside of the control electrodestructures 306A and 306B and gate electrode structures 307A, 307B. Theselective removing of the insulating layer 204 may for example beprovided simultaneous with an etching of the hard mask used forstructuring the control electrode structures 306A and 306B and gateelectrode structures 307A, 307B.

With reference to FIG. 3D, a layer 308 is deposited. In the describedembodiment, the layer 308 overlays the control electrode structures andgate electrode structures as well as the regions between adjacentcontrol electrode structures and the regions between adjacent gateelectrode structures. In embodiments, the layer 308 may be a layerdeposited conformal and continuous over the structures of the substrate202. In embodiments, the layer 308 provides protection for the substrate202 in the pixel region between the control electrode structures 306Aand 306B during a spacer etch back as will be described further below.This allows to achieve improved conversion efficiency in the pixelregions since the concentration of impurities or other defects introduceby the spacer etch back is reduced significantly in the region extendingbetween the control electrode structures 306A and 306B. The layer 308 isremoved in the transistor area to allow the formation of spacerstructures and a substrate doping by using the spacer structures to formtransistors. Since in embodiments the protection layer remains in thepixel region between adjacent control electrodes during all furtherprocessing steps or at least some further processing steps, the layer308 further protects the substrate interface in the pixel area fromcontamination and damages also for the following processing steps. Forexample, protection in the pixel area from contamination of thesubstrate interface by dopants used in doping the transistor area can beachieved. Therefore, the number of recombination centers existing at thesubstrate interface in the pixel area which reduce an efficiency of thedevice can be reduced compared to a process in which the pixel region isnot protected by the layer 308 and therefore subjected to the spaceretch back or dopants in the same way as the transistor region. The layer308 therefore allows to use the same processing steps for forming thecontrol structures in the pixel region and for forming the controlstructures in the transistor region without damaging the substrateinterface in the pixel area by the processing steps needed in thetransistor area for example to obtain prober doping by using the spacerstructures.

The layer 308 provides in addition synergetic effects in manufacturingthe device since electric effects which are caused by permanentlycharged layers formed later on in the manufacturing process above thesubstrate are weakened in view of the additional distance provided bythe layer 308 to these charged layers. Charged layers may influence forexample through their electric field photo-generated charge carriers andmay for example reduce the number of photo-generated charge carrierswhich reach the influence zone of the control electrodes 208A, 208Bclose to the substrate interface. Thus, the efficiency of a pixelmanufactured by embodiments herein described can further be enhanced.Overall, a pixel for an imager device can be formed with improvedefficiency since less photo-generated charge carriers are recombining atimpurities or defects. This may for example be of significant importancewhen shrinking down to small sized pixel areas in current and futurehigh density pixel depth imagers. Better protection is achieved whenlayer 308 is made thick. However thickness of the layer 308 isrestricted in view of the modern processing techniques and small sizeddistances between adjacent control electrode structures. For typicalmodern CMOS-processing techniques, a range of thickness from 10 nm(nanometer) up to 100 nm allows to obtain good results in efficiency. Insome embodiments, the range of thickness may be between 20 and 50 nmallowing the manufacturing of small sized pixels with sufficientprotection. In some embodiments, the layer 308 comprises oxide materialsuch as silicon oxide. Oxide material may provide additional synergeticeffects in manufacturing since permanent charging effects duringdeposition within the layer are reduced. As outlined above, suchcharging effects may affect the photo-generated charge carriers.

As described above, the layer 308 extends at least in the region betweenthe control electrode structures 306A and 306B in order to provideprotection for the substrate interface. However as shown in FIG. 3D, thelayer 308 may also overlay the control electrode structures 306A and306B. In other embodiments of more than two control electrode structuresper pixel, the layer 308 may be deposited in each region extendingbetween two adjacent control electrode structures. In some embodiments,the layer 308 may extend in each region between two adjacent controlelectrode structures and further overlaying each of the controlelectrode structures.

With reference to FIG. 3E, the layer 308 is then completely removed inthe transistor region.

As shown in FIG. 3F, a spacer layer 310 is formed. The spacer layer 310extends in the pixel area directly interfacing the layer 308 while thespace layer extends in the transistor region interfacing the substratein the region between the gate electrode structures 307A, 307B andinterfacing the top and sidewall surfaces of the gate electrodestructures 307A, 307B. Thus as can be seen in FIG. 3F, the layer 308 isin the pixel region sandwiched between the surfaces of the controlelectrode structures 306A, 306B and the spacer layer 310. In the regionbetween the control electrode structures 306A, 306B, the layer 308 issandwiched between the substrate 202 and the spacer layer 310.

In some embodiments, the spacer layer 310 may comprise at least firstand second sublayers or more than 2 sublayers. The first and secondsublayers may have a same material or may have different materials. Insome embodiments, similar to layer 308, a further layer 312 may beformed between the first and second sublayers of spacer layer 310 toprovide protection as described above. In some embodiments, instead offorming the layer 308 to be below the spacer layer 310, the layer 308may be formed between sublayers of a spacer layer. In some embodiments,the spacer layer 310 includes oxide or nitride material. In someembodiments, in which multiple sublayers are used and each sublayer isetched back, the sublayer directly interfacing the layer 308 has amaterial different than the material of the layer 308 while othersublayers not directly interfacing the layer 308 may have a samematerial as layer 308.

Referring now to FIG. 3G, the spacer layer 310 is etched back togenerate spacer structures 312 at sidewalls of the control electrodestructures 306A, 306B. As outlined already above in detail, during theetch back, the layer 308 protects the substrate region between thecontrol electrode structures 306A and 306B. In some embodiments of aspacer layer with sublayers, each sublayer may be subjected to an etchback process while the layer 308 still protects the substrate region inthe pixel region between the control electrodes 306A, 306B. Then adoping may be performed to obtain doped regions in the transistor area.Again in view of the layer 308 maintained in the pixel area, thesubstrate is protected in the pixel area from being contaminated bydopants without having to apply any additional protection layers.

Afterwards the manufacturing of the device is continued as is known to aperson skilled in the art including the finishing of the FEOL (front endof line), MEOL (middle end of line) and BEOL (Back end of line).

Referring now to FIGS. 4A-4I, a further embodiment is described. In thisembodiment, a first spacer layer is deposited before the layer 308 isgenerated and structured.

FIGS. 4A to 4C are identical with FIGS. 3A to 3C and therefore referenceis made to the detailed description above.

With reference to FIG. 4D, a first spacer layer 402 is depositedconformal above the structures and the substrate in the pixel region andin the transistor region. As shown in FIG. 4E, the layer 308 is thendeposited on the first spacer layer 402. Then the layer 308 is removedin the transistor region for example by selective etching against thefirst spacer layer 402. FIG. 4F shows the resulting structure in whichthe layer 308 extends above the first spacer layer 402 in the pixelregion while the layer 308 is removed in the transistor region. A firstspacer etch back is applied which forms spacer structures 404 at leastat the side walls of the gate electrodes 307A 307B. While FIG. 4G showsthe spacer structures 404 at side walls of the gate electrode structures307A, 307B, it is to be understood that parts of the spacer layer 402may also remain in other areas, for example above the gate electrodestructures 307A, 307B. It is further to be noted that in view of thelayer 308 not removed in the pixel region and covering the spacer layer402, the spacer layer 402 provides protection for future processingsteps in the pixel region.

With reference to FIG. 4H, after the etching back of the first spacerlayer 402, a second spacer layer 406 is deposited in the pixel regionand the transistor region. The second spacer layer 406 is etched backand forms spacer structures 408 at sidewalls in the pixel region and thetransistor region which are shown in FIG. 4I. With the spacer structures404 and 408 formed in the transistor region, a doping is performed togenerated doped regions in the transistor regions. Other processingsteps are provided to continue with the further processing formanufacturing the imager device.

It is to be understood that each of the spacer layers 404 and 408 mayinclude in other embodiments multiple sublayers of a same or differentmaterial. Thus, in some embodiments each of the sublayers may beseparately deposited and subjected to an etch-back in order to formadditional spacer structures.

After the spacer structures have been formed, additional processingincludes the manufacturing of a metal stack in the BEOL. Typically, apassivation layer is provided on top of the metal stack for protectionpurposes.

Embodiments described herein may use instead of a single passivationlayer at least two passivation layer.

According to some embodiments, the passivation layer on top of the metalstack can be provided in a manner to provide in addition antireflectioncoating behavior. According to some embodiments which are describedbelow, in order to achieve an antireflection coating behavior of thepassivation layer, the passivation layer has a reduced thickness in thelight sensitive area compared to other areas. To achieve this, thepassivation layer may include a multiple-layer stack where at least onelayer is removed in the light sensitive areas to allow a matching of thepassivation layer in the light sensitive area to achieve antireflectivecoating behavior.

FIG. 5A shows a first embodiment in which above the substrate 202 ametal layer stack 502 is generated. The layer stack 502 comprises aplurality of metal layers 504 which are embedded in layers 505. Betweeneach metal level an electrical insulating layer 506 which may comprisenitride material is formed. First and second passivation layers 508 and510 extend in areas outside of the light sensitive areas. In the lightsensitive areas, the first passivation layer is removed such that onlythe second passivation layer is provided having a thickness matched foranti-reflective behavior. In some embodiments, in addition to removingthe first passivation layer, also a part of the metal stack may beremoved. This avoids reflection of the incoming light whichsubstantially take place at the isolation layers of the metal levels.The removal of the metal stack avoids that a significant part of thelight does not reach the light sensitive region in the substrate and islost for the signal.

FIG. 5B shows an embodiment in which a part of the metal stack isremoved in the light sensitive areas to reduce the reflection andabsorption of light in the metal stack. As can be seen from FIG. 5B,only the second passivation layer 510 is provided in the area in whichthe metal stack is removed FIG. 5B shows the upper three of the layers505 in which the metal layers 502 are formed being removed while threeother layers 505 are remaining above the light sensitive area of thesubstrate 202.

FIG. 5C shows an embodiment, in which all metal stack layers are removedin the light sensitive areas. Here the second passivation layer 510extends on or close to the substrates surface, for example at a maximumdistance from the substrate which is smaller than the vertical extensionof the lowest metal level.

The embodiments shown in FIG. 5A-C may alone or in combination with themanufacturing process steps shown in FIGS. 3A-G and 4A-I provide asignificant improvement of the efficiency of the imager device.

In some embodiments, a lens may be formed for each pixel. The lens maybe formed from a photo-sensitive resist material such as Durimide. Suchlenses have been tested to have good optical spectral behavior forinfrared light which might be used for example in the depth imager shownin FIG. 2A. The photo-sensitive resist material allows convenientforming of the lenses by having the polyimide selectively irradiated.When applying a developer solution, the non-irradiated portions areremoved. After the application of the developer the remaining structureis hard baked for example at about temperatures between 80 and 150° C.

A further improvement of efficiency of the depth imager 200 can beobtained in some embodiments by providing for each pixel a reflectionopposite to a respective lens provided for each pixel. The reflectivestructure provides a mirror for non-absorbed light and reflects back thelight to concentrate in the pixel's photo-generation region. FIG. 6Ashows an example in which a concave structure 602 is formed on the backside of the substrate 202 which is mounted to an external substrate 604such as a glass substrate. Within the concave structure 602, a convexpart 603 is formed which provides the focusing effect for the lightreflected back to the photo-generation region. The convex part 603 maybe fully or partially overlaid with a reflective layer such as a metallayer to provide good reflection. Light not absorbed in the lightabsorbing region is reflected by the reflective layer having a convexshape and directed back to the light absorbing regions in a focusingmanner. In a further embodiment shown in FIG. 6B, the concave structure602 may be formed on a separate substrate such as a glass plate on whichthe substrate 202 is mounted. A metal layer of concave shape is formedon a part or fully overlaying the concave structure 602. Light notabsorbed in the light absorbing region is reflected by the reflectivelayer having a concave shape and directed back to the light absorbingregions in a focusing manner. It is to be understood that theembodiments of reflective structures shown in FIGS. 6A and 6B may becombined with any of the above described embodiments described withrespect to FIGS. 3A-G, 4A-I and 5A-C.

Further enhancement in the sensitivity for depth imager sensors can beobtained by using a new doping concept as described below. In general,semiconductor doping technology is based on implementing differentlydoped (n and p) regions in order to provide a locally definedconductivity. Within n-regions, typically SB, AS or P, for p-regions,typically B (BF2) is doped into the substrate. Most significant is theion implantation in which an accelerated atom stream is guided andintroduced to the substrate. The area dose can be exactly defined bystream integration. After the implantation, the formerly perfect crystalis however severely damaged such that a usage is not immediatelypossible and annealing of the crystal at temperatures typically above1000° C. and over many hours is required. A perfect annealing is howevernot possible and damages have to be tolerated. The higher the dose, theenergy and the AMU (atomic molecular unit) is, the more damages occurand the more complex the damages are. Such damages typically result inleakage currents and other problems.

The present concept for obtaining a p-doping with improved performanceand pixel efficiency for depth imagers and 3D sensors utilizes adifferent approach in which doping is achieved by a diffusion from a Bordoped glass layer (e.g. SiO2+B). The Bor doped glass layer is depositedover the substrate previous to the diffusion. In a reactor capable ofsilicon dioxide layer deposition, gas comprising Bor is added whichresults in a SiO2 layer comprising Bor. In some embodiments, the maximumconcentration of Bor in the SiO2 layer may be below 20%, in someembodiments the maximum concentration of Bor in the SiO2 layer may bebelow 15% in some embodiments, the maximum concentration of Bor in theSiO2 layer may be at a value between 15% and 20%. FIG. 7A shows thesubstrate 202 prior to the doping. A layer 702 of Bor doped SiO2 isdeposited over the substrate 202. Layer 702 may for example be depositedin a silicon dioxide reactor in which Bor gas is added. After thedeposition of the Bor-doped silicon dioxide layer, the Bor-dopants aredriven from the Bor-doped glass layer to the substrate 202. A heatingprocess applied at temperatures above 1000° C. and for many hours istypically used to drive the Bor-dopants from the Bor-doped glass layerin the substrate 202. FIG. 7B shows the substrate after the drive-inprocess. A region 202A of high p-doping is generated in the substrate202. The doped region 202A may have in some embodiments a cumulateddoping concentration of 1015 cm-2 or more. In some embodiments, a peakdoping concentration within the region 202A may have a value of 1019cm-2 or more. In some embodiments, the peak doping concentration mayinclude a value in the range between 1019 cm-2 and 1020 cm-2. Inembodiments, the SiO2 layer may be removed afterwards and an epitaxiallayer is grown above the highly doped substrate 202.

FIG. 8 shows a doping profile 802 with a dose of 1×10⁶ cm⁻² resultingfrom diffusion from a Bor concentration of 4% within the SiO2 layer over4 hours at a temperature of 1050° C. The ordinate of FIG. 8 show dopingconcentrations on a logarithm scale and the abscissa of FIG. 8 shows thelayers extension in μm. For comparison, a doping profile with a dose of4.7×10¹⁴ cm⁻² resulting from an implant doping is shown in FIG. 8. Thestructure shown in FIG. 8 includes the substrate 202 with the dopedregion 202A and an epitaxial layer 202B grown above the doped region202A after the respective doping processes have been applied. It isapparent that the doped region 202A being after the deposition ofepitaxial layer 202B sandwiched at both sides by crystal semiconductormaterial forms in the structure of FIG. 8 a buried layer with highdoping concentration. Such structures may be provided in someembodiments to obtain a specific doping profile such as, but notrestricted to, a doping profile to generate build-in fields as describedin co-owned U.S. provisional application 61/731,373, the content ofwhich is incorporated herein in its full entirety.

It can be seen in FIG. 8 that the doping profile 802 resulting from thediffusion of the Bor doped SiO₂ glass layer 702 achieves a higher dopingconcentration with a peak doping concentration of almost up to 1020 cm⁻²compared to a profile 804 resulting from the implanted doping. Thedoping concentration with Bor doped SiO₂ layers thus allows achievingp-doping concentrations which are significantly above the p-dopingconcentrations which can be achieved by doped ion implantation withreasonably crystal defects. In addition, in view of the smoother methodof doping, crystal defects are reduced resulting in less trapping ofcharge carriers and higher quantum efficiency for the imager.

FIG. 9 shows a diagram wherein a relation of Bor doping concentrationsin the Bor doped SiO₂ layer before diffusion (shown as weight percentageof SiO₂ on the abscissa) and Bor doping concentrations which areachieved after diffusion in the silicon substrate (shown in 1/cm² on theordinate in logarithmic scale) is shown. In FIG. 9, a curve 902 shows acumulative Bor dose concentration in silicon and a curve 904 shows apeak Bor doping concentration in silicon. It can be observed that ratiosof peak concentration to cumulative dose (c_peak/c_cumul) can beachieved with values up to 4 decades (104) in a range of cumulative doseconcentrations between about 8×10¹⁴ cm⁻² and 2×10¹⁶ cm⁻² (peakconcentrations between about 6×10¹⁸ cm⁻² and 1×10²⁰ cm⁻²). The range ofpeak and cumulative Bor concentrations in the silicon can be varied byvarying the concentration of Bor in the SiO₂ layer glass layer from 1%to 4.5%. to achieve cumulative dose concentrations between about 8×10′⁴cm⁻² and 2×10¹⁶ cm⁻² and peak concentrations between about 6×10¹⁸ cm⁻²and 1×10²⁰ cm⁻². For comparison, a cumulative Bor dose concentrationachievable with ion implantation is shown in the diagram of FIG. 9 withreference number 906 and a peak Bor concentration achievable with ionimplantation is shown with reference number 908.

It becomes clear that the above described doping concept interacts in asynergetic manner with the described embodiments to avoid crystaldefects which would increase light trapping and reduce efficiency of thedepth imager devices. In particular, the concept for avoiding additionaldamages of the crystal semiconductor material during the furthermanufacturing such as described in FIGS. 3A to 3G provides a synergisticeffect with the above described doping concept.

In the above description, embodiments have been shown and describedherein enabling those skilled in the art in sufficient detail topractice the teachings disclosed herein. Other embodiments may beutilized and derived there from, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. For example, while a manufacturing concept of an 3DTOF imager based on the photonic mixing principle has been described, itmay be understood that the above concepts can also applied to themanufacturing of other 3D imagers such as 3D TOF imagers which are notbased on the photon mixing concept. Furthermore, it may be understoodthat the described manufacturing concept may also be applicable toimagers in general which include for example also 2D imagers.

This Detailed Description, therefore, is not to be taken in a limitingsense, and the scope of various embodiments is defined only by theappended claims, along with the full range of equivalents to which suchclaims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

It is further to be noted that specific terms used in the descriptionand claims may be interpreted in a very broad sense. For example, theterms “circuit” or “circuitry” used herein are to be interpreted in asense not only including hardware but also software, firmware or anycombinations thereof. The term “data” may be interpreted to include anyform of representation data. The term “information” may in addition toany form of digital information also include other forms of representinginformation. The term “entity” or “unit” may in embodiments include anydevice, apparatus circuits, hardware, software, firmware, chips or othersemiconductors as well as logical units or physical implementations ofprotocol layers etc. Furthermore the terms “coupled” or “connected” maybe interpreted in a broad sense not only covering direct but alsoindirect coupling.

It is further to be noted that embodiments described in combination withspecific entities may in addition to an implementation in these entityalso include one or more implementations in one or more sub-entities orsub-divisions of said described entity. For example, specificembodiments described herein described herein to be implemented in atransmitter or receiver may be implemented in sub-entities such as achip or a circuit provided in such an entity.

The accompanying drawings that form a part hereof show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced.

In the foregoing Detailed Description, it can be seen that variousfeatures are grouped together in a single embodiment for the purpose ofstreamlining the disclosure. This method of disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter lies in lessthan all features of a single disclosed embodiment. Thus the followingclaims are hereby incorporated into the Detailed Description, where eachclaim may stand on its own as a separate embodiment. While each claimmay stand on its own as a separate embodiment, it is to be notedthat—although a dependent claim may refer in the claims to a specificcombination with one or more other claims—other embodiments may alsoinclude a combination of the dependent claim with the subject matter ofeach other dependent claim. Such combinations are proposed herein unlessit is stated that a specific combination is not intended. Furthermore,it is intended to include also features of a claim to any otherindependent claim even if this claim is not directly made dependent tothe independent claim.

Furthermore, it is intended to include in this detailed description alsoone or more of described features, elements etc. in a reversed orinterchanged manner unless otherwise noted.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective steps of these methods.

Further, it is to be understood that the disclosure of multiple steps orfunctions disclosed in the specification or claims may not be construedas to be within the specific order. Therefore, the disclosure ofmultiple steps or functions will not limit these to a particular orderunless such steps or functions are not interchangeable for technicalreasons.

Furthermore, in some embodiments a single step may include or may bebroken into multiple substeps. Such substeps may be included and part ofthe disclosure of this single step unless explicitly excluded.

While the foregoing has been described in conjunction with exemplaryembodiment, it is understood that the term “exemplary” is merely meantas an example, rather than the best or optimal. Accordingly, thedisclosure is intended to cover alternatives, modifications andequivalents, which may be included within the scope of the disclosure.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present disclosure. This disclosure isintended to cover any adaptations or variations of the specificembodiments discussed herein.

What is claimed is:
 1. A method of manufacturing a depth imager,comprising: providing a substrate having a photo-conversion region;forming a metal layer stack on a front side of the substrate; providingan optical microlens that focuses light towards the photo-conversionregion of the substrate; and mounting the substrate onto a separateexternal glass substrate, the external glass substrate working inconjunction with a concave reflection structure to reflect non-absorbedlight back to the photo-conversion region of the substrate.
 2. Themethod of claim 1, further comprising: forming the concave reflectionstructure on a back side of the substrate.
 3. The method of claim 1,further comprising: forming a convex part within the concave reflectionstructure, the convex part providing a focusing effect for the reflectednon-absorbed light.
 4. The method of claim 3, further comprising:overlaying a reflective layer onto the convex part.
 5. The method ofclaim 1, further comprising: forming the concave reflection structure onthe external glass substrate.
 6. The method of claim 1, furthercomprising: forming a concave-shaped metal reflection layer onto theconcave reflection structure, the concave-shaped metal reflection layerreflecting the non-absorbed light back to the photo-conversion region ofthe substrate.
 7. The method of claim 1, wherein providing the opticalmicrolens includes forming the optical microlens from a photo-sensitiveresist material.
 8. The method of claim 1, wherein providing the opticalmicrolens includes forming the optical microlens from polyimide.
 9. Themethod of claim 1, wherein providing the optical microlens includesforming the optical microlens by selective irradiation.
 10. The methodof claim 9, wherein forming the optical microlens by selectiveirradiation includes forming the optical microlens by applying adeveloper solution to remove a non-irradiated portion of polyimide, andthen hard-baking the remaining structure.
 11. A depth imager,comprising: a substrate having a photo-conversion region; a metal layerstack on a front side of the substrate; an optical microlens configuredto focus light towards the photo-conversion region of the substrate; andan external glass substrate separate from the substrate onto which thesubstrate is mounted, wherein the external glass substrate is configuredto work in conjunction with a concave reflection structure to reflectnon-absorbed light back to the photo-conversion region of the substrate.12. The depth imager of claim 11, wherein the concave reflectionstructure is formed on a back side of the substrate.
 13. The depthimager of claim 11, further comprising: a convex part formed within theconcave reflection structure, the convex part being configured toprovide a focusing effect for the reflected non-absorbed light.
 14. Thedepth imager of claim 13, further comprising: a reflective layeroverlaid onto the convex part.
 15. The depth imager of claim 11, whereinthe concave reflection structure is formed on the external glasssubstrate.
 16. The depth imager of claim 11, further comprising: aconcave-shaped metal reflection layer formed onto the concave reflectionstructure, the concave-shaped metal reflection layer being configured toreflect the non-absorbed light back to the photo-conversion region ofthe substrate.
 17. The depth imager of claim 11, wherein the opticalmicrolens is formed from a photo-sensitive resist material.
 18. Thedepth imager of claim 11, wherein the optical microlens is formed frompolyimide.
 19. The depth imager of claim 11, wherein the opticalmicrolens is formed by selective irradiation.
 20. The depth imager ofclaim 11, wherein the depth imager is a time-of-flight (TOF) depthimager, and further comprising: a TOF depth imager circuit.